The internal surface of a conventional metal can is generally coated with a thin protective film of a synthetic resin for preventing contents from directly contacting and corroding a metal material such as tin, tin-free steel or aluminum.
The coating composition for coating the internal surface is generally selected from solvent-type epoxy coating compositions such as epoxy-phenol, epoxy-amino and epoxy-acryl coating compositions.
However, the above coating composition has a defect that they are poor in processability, since they are basically formed of thermosetting resins.
For the above reason, in the kind and filed of cans which are required to have processability in particular, there is no other choice but to use vinyl chloride resin-based organosol coating composition which is basically formed of a thermoplastic resin and has a defect that a large amount of an extract is caused and that it has poor resistance to flavor alteration.
Meanwhile, the use of an epoxy/aryl water-based coating composition is beginning in recent years for workability, environmental purposes or demands for safety (conversion to danger-free articles).
The following coating compositions have been so far disclosed; a water-based coating composition obtained by esterifying an epoxy resin with an acrylic resin containing a carboxyl group, neutralizing the resultant ester with a base and dispersing the ester in water (JP-B-59-37026), a water-based coating composition obtained by copolymerizing an epoxy resin having an acryloyl group with an ethylenically unsaturated monomer containing acrylic acid or methacrylic acid and dispersing the resultant copolymer in water in the same manner as that described above (JP-B-62-7213), and a water-based coating composition obtained by grafting a polymerizable monomer mixture containing a carboxyl group to an epoxy resin in the presence of a free radical generating agent and dispersing the resultant polymer in water in the same manner as that described above (JP-B-63-17869).
The above coating compositions are self-emulsifiable epoxy resin water-based coating composition of which the resin itself exhibits dispersibility, and they do not contain any surfactant for dispersing them in water. The above coating compositions are therefore excellent in chemical performance and water resistance.
All of the above self-emulsifiable epoxy resin water-based coating compositions contain an epoxy resin which is modified with an acrylic resin, and there is a defect that they do not easily exhibit the performances inherent to the epoxy resin, such as adhesion to an undercoating, corrosion resistance and processability. In particular, an epoxy resin having a high molecular weight does not easily undergo self-emulsification, and there is a defect that the high processability thereof is impaired.
It is an object of the present invention to provide a water-based coating composition which overcomes various problems of the above conventional vinyl chloride resin-based organosol coating composition and self-emulsifiable epoxy resin water-based coating composition and which is excellent particularly in processability and corrosion resistance.